This invention relates to cyclical swing adsorption processes for the separation of a feed gas mixture. As used in this specification, the term “separation” includes the removal of contaminants and/or impurities from a gas stream that may subsequently be further separated. The invention has particular, but not exclusive, application to removing, or at least reducing the level of, carbon dioxide in a feed gas to render it suitable for downstream processing. The invention is especially useful in removing carbon dioxide from air which is to be employed as a feed gas in a process for the cryogenic separation or purification of air.
Carbon dioxide is a relatively high boiling gaseous material and removal of this and other high boiling materials, for example water, which may be present in a feed gas is necessary where the mixture is to be subsequently treated in a low temperature, for example cryogenic, process. If relatively high boiling materials are not removed, they may liquefy or solidify in subsequent processing and lead to pressure drops and flow difficulties in the downstream process. It may also be necessary or desirable to remove hazardous, for instance explosive, materials prior to further processing of the feed gas so as to reduce the risk of build-up in the subsequent process thereby presenting an explosion hazard. Hydrocarbon gases, for example acetylene, may present such a hazard.
Several processes are known for separating one or more components from a feed gas mixture using selective adsorption by a solid adsorbent. These processes include temperature swing adsorption (TSA), pressure swing adsorption (PSA), thermal pressure swing adsorption (TPSA) and thermally enhanced pressure swing adsorption (TEPSA). Usually, the process is conducted in a cyclical manner in which one adsorber bed is in an on-stream mode, during which adsorbate is adsorbed from a feed gas mixture passing through the bed, while another adsorber bed is in a regeneration mode, during which the adsorbed adsorbate is desorbed from the bed, and said beds alternate between said modes,
Generally, in these processes having air as the feed gas, water and carbon dioxide are removed from an air feed gas by contacting the mixture with one or more adsorbents which adsorb water and carbon dioxide. The water adsorbent material typically is silica gel, alumina or a molecular sieve and the carbon dioxide adsorbent material typically is a molecular sieve, for example, a zeolite. It is conventional to remove water first and then carbon dioxide by passing the feed air through a single adsorbent layer or separate layers of adsorbent selected for preferential adsorption of water and carbon dioxide in a column. Removal of carbon dioxide and other high boiling components to a very low level is especially desirable for the efficient operation of downstream processes.
After adsorption, the flow of feed gas is shut off from the adsorbent bed and the adsorbent is exposed to a flow of regeneration gas which strips the adsorbed materials, for example carbon dioxide and water, from the adsorbent and so regenerates it for further use.
In a TSA process for carbon dioxide and water removal, atmospheric air is typically compressed using a main air compressor (MAC) followed by water-cooling and removal of the thus condensed water in a separator. The air may be further cooled using, for example, refrigerated ethylene glycol. The bulk of the water is removed in this step by condensation and separation of the condensate. The gas is then passed to an adsorber bed system where the remaining water and carbon dioxide are removed by adsorption.
By using two adsorbent beds in a parallel arrangement, one may be operated for adsorption while the other is being regenerated and their roles periodically reversed in the operating cycle. Conventionally equal periods are devoted to adsorption and to regeneration.
As the component which is being removed from the feed gas is adsorbed while the bed is on-stream, the adsorption process will generate heat of adsorption causing a heat pulse to progress downstream through the adsorbent. The heat pulse is allowed to proceed out of the downstream end of the adsorbent bed during the feed or on-stream period. During the regeneration process, heat must be supplied to desorb the gas component which has been adsorbed on the bed. In the regeneration step, part of the product gas, for instance nitrogen or a waste stream from a downstream process, is used to desorb the adsorbed components and may be compressed in addition to being heated. The hot gas is passed through the bed being regenerated so removing the adsorbate. Regeneration conventionally is carried out in a direction counter to that of the adsorption step.
In a PSA system, cycle times are usually shorter than in a TSA system, but feed temperature and pressure and the regeneration gas often are similar. However in PSA systems, the pressure of the regeneration gas is lower than that of the feed gas and the change in pressure is used to remove the carbon dioxide and water from the adsorbent. Regeneration is suitably commenced before the heat pulse mentioned above in relation to TSA has reached the downstream end of the bed. The direction of the heat pulse is reversed by the process of regeneration and the heat which derived from the adsorption of the gas component in question is retained in the bed and used for desorbing that component during regeneration. In contrast to TSA, it is unnecessary to heat the regeneration gas.
Thermal pressure swing adsorption (TPSA) is also suitable for removing carbon dioxide and water from feed air. In a TPSA system, water is typically confined to a zone in which a water adsorption medium, for example activated alumina or silica gel, is disposed. A separate layer comprising, for example, a molecular sieve for the adsorption of carbon dioxide is typically employed and the molecular sieve layer and the zone for adsorption of water conventionally are separate. In contrast to a TSA system, water does not enter the molecular sieve layer to any significant extent which advantageously avoids the need to input a large amount of energy in order to desorb the water from the molecular sieve layer. TPSA processes are described in, for example, U.S. Pat. Nos. 5,885,650 and 5,846,295, the contents of which are incorporated herein by this reference.
Thermally enhanced PSA (TEPSA), like TPSA, utilizes a two stage regeneration process in which carbon dioxide previously adsorbed is desorbed by TSA and adsorbed water is desorbed by PSA. In this process, desorption occurs by feeding a regeneration gas at a pressure lower than the feed stream and a temperature greater than the feed stream and subsequently replacing the hot regeneration gas by a cold regeneration gas. The heated regenerating gas allows the cycle time to be extended as compared to that of a PSA system so reducing switch losses as heat generated by adsorption within the bed may be replaced in part by the heat from the hot regeneration gas. A TEPSA process is described in, for example, U.S. Pat. No. 5,614,000, the content of which is incorporated herein by this reference.
In contrast to PSA, TSA, TEPSA and TPSA all require the input of thermal energy by means of heating the regeneration gas but each procedure has its own characteristic advantages and disadvantages. The temperatures needed for the regenerating gas are typically sufficiently high, for example 50° C. to 200° C., as to place demands on the system engineering which increases costs. Typically, there will be more than one adsorbate which is removed in the process and generally one or more of these components, for example water, will adsorb strongly and another, for example carbon dioxide, much more weakly. The high temperature used for regenerating needs to be sufficient for the desorption of the more strongly adsorbed component. The high temperature employed in a TSA, TPSA and TEPSA systems may require the use of insulated vessels, a regeneration gas preheater and an inlet end precooler, and generally the high temperatures impose a more stringent and costly mechanical specification for the system. In operation, there is extra energy cost associated with using the purge preheater. The PSA system avoids many of these disadvantages by avoiding the need for coping with high temperatures, although the short cycle time which characterizes PSA brings its own disadvantages.
The design of a swing adsorption system takes account of potential variations in the composition of the feed gas mixture to be separated and conventionally is based on the worst possible feed conditions to accommodate all of the potential variations. Usually, the process conditions for the system are pre-selected and remain constant during operation in order to ensure that the feed gas having the highest likely content of adsorbate may be processed without risk of exceeding the capacity of the system to remove the adsorbate and so avoiding unacceptable levels of adsorbate being passed to a downstream process. In the case of removal of carbon dioxide and water from air, account is taken of the ambient prevailing conditions in the locality in which the process is to be operated as the level of carbon dioxide changes according to pollution levels and water in the feed gas changes according to variations in local temperature and relative humidity. In the particular case of carbon dioxide pollution, the carbon dioxide content of feed air can change rapidly and substantially in response to a change in wind direction if there is a burner stack emitting carbon dioxide in the vicinity or to a change in local weather conditions. For example, FIG. 1 is a chart showing the variation in ambient carbon dioxide concentration at an Air Products' air separation unit at Wigan, UK during the period 20th to 23rd Nov. 2005. There was foggy weather on 20th to 22nd Nov. 2005 during which the carbon dioxide concentration was above the normal level of about 450 ppm and reached a peak of about 680 ppm. Similarly, FIG. 2 is a chart showing the variation in ambient carbon dioxide concentration at an Air Products' air separation unit at Isle of Grain, UK during the period 4th to 8th Sep. 2006. There is a LNG burner stack in the vicinity of the unit and the effect of carbon dioxide emitted by that stack on the air separation unit is dependent upon the wind direction. As can be seen, carbon dioxide concentration peaked to over 10000 ppm.
There have been proposals in the prior art to vary the cycle time of a cyclical swing adsorption process to accommodate changes in feed gas composition. For example, U.S. Pat. No. 3,808,773 discloses the adsorptive purification of a gas containing water and one or more secondary components by passing the gas through a molecular sieve bed to remove adsorbable components, terminating the gas flow prior to the breakthrough of adsorbate water vapor therefrom, preferably upon the breakthrough of the least easily adsorbable secondary component, and then regenerating the molecular sieve at a relatively low temperature of 100-200° C. by passing a heated purge gas through the molecular sieve in the opposite direction to the gas flow. A dual bed system is described in which adsorption is conducted in one bed whilst the other undergoes regeneration for a set time period.
U.S. Pat. No. 4,197,095 discloses the adsorption of component(s) from a gas feed using a dual bed adsorption process in which operating conditions including the flow rate, inlet and outlet temperatures, inlet and outlet pressures and regenerating pressures are sensed; the quantity of purge flow required to regenerate the bed calculated; the purge flow rate under the operating conditions calculated; and the regeneration time controlled so that the purge flow stops when the bed has been regenerated. The cycling time is controlled at a period not shorter than the regeneration time and the beds switch at the end of that time.
U.S. Pat. No. 4,472,178 discloses the adsorption of carbon dioxide from a water-depleted gas feed stream gas stream by a TSA sequence in which the carbon dioxide-depleted gas product of an adsorption bed is initially passed through a recuperator to retain heat but, after the gas reaches a predetermined lower temperature, bypasses the recuperator. The flow of feed gas is discontinued when the gas product reaches a predetermined carbon dioxide concentration, the pressure of the bed is reduced and the bed initially purged countercurrently with a purge gas, which has been heated by externally supplied heat and recovered heat from the recuperator, until a thermal zone exists in the bed. The purge is continued without externally supplied heat until the thermal zone is approximately at the feed end of the bed and then discontinued, the bed is repressurized countercurrently with water and carbon dioxide-depleted gas until the bed reaches a preset pressure such that the adsorption cycle can be initiated again. A dual bed system is described in which adsorption is conducted in one bed whilst the other undergoes regeneration for a set time period.
U.S. Pat. No. 4,693,730 discloses a pressure swing adsorption process in which a characteristic of the effluent from cocurrent depressurization is sensed and corrective action responsive thereto taken to control product purity. The action can be adjustment of the amount of purging gas received by an adsorbent bed to control the extent of regeneration. In the exemplified embodiment, the sensed characteristic is impurity concentration and cycle times and impurity level target values are adjusted and the effluent characteristic of the depressurization of one bed results in corrective action affecting all beds.
U.S. Pat. No. 5,989,313 discloses PSA prepurification of air in which the cycle time for each of at least two adsorbers is controlled by a “real time” method in which actual totalized flow to an on-stream adsorber is accumulated, based upon measured flow values over a predetermined time period, and air feed conditions, for example, temperature, pressure, relative humidity, are monitored. Periodically, a maximum totalized flow to the adsorber is calculated based upon the monitored air feed conditions, the actual totalized flow value compared to the current calculated maximum totalized flow value and, when a predetermined relationship therebetween is reached, the on-stream adsorber is decoupled from the air feed and another adsorber is coupled thereto. The control of cycle times for each adsorber also takes into consideration: variations in load demand, purge to air feed ratio and upsets which occur on switching of adsorber beds. The purge to air feed ratio can be controlled, based on air flow and column recovery, and also if the bed temperature is high, such as in summer, the purge flow can be reduced.
U.S. Pat. No. 6,277,174 discloses a PSA process in which the maximum feed pressure to each of at least two beds is monitored during adsorption and the minimum evacuation pressure from each said bed is monitored during desorption, and individual step times are altered within a cycle, in accordance with the monitored pressures, to control flows to and between the beds to maintain a nearly constant pressure ratio. Purge and overlap equalization step times can be adjusted in accordance with the monitored pressures.
U.S. Pat. No. 6,402,809 discloses the purification of a gas, such as air, containing carbon dioxide and/or water by a TSA process in which at least one energy parameter, chosen from the flow rate of the regeneration gas entering and/or leaving an adsorber, the duration of the regeneration step and the regeneration temperature of the regeneration gas entering the adsorber, is controlled, modified and/or regulated depending on at least one operating condition chosen from the pressure of the gas to be purified entering and/or leaving the adsorber, the flow rate of the gas to be purified entering and/or leaving the adsorber, the temperature of the gas to be purified entering the adsorber and the content of impurities contained in the gas to be purified entering the adsorber and depending on the thermal profile of the heat front output by the adsorber at the end of regeneration. Preferably adsorption is conducted in one bed whilst another bed undergoes regeneration.
U.S. Pat. No. 6,599,347 discloses the adsorption of water and carbon dioxide from a feed gas using a thermal swing adsorption process in which one or more parameters relating to the water content of the feed gas is determined directly or indirectly and the adsorbent is regenerated using conditions based on said parameter(s). The feed gas parameter(s) can be measured continuously or periodically such as hourly or daily and the purge gas flow and/or temperature modified in response to the measured data.
None of these prior art processes permit of automatic control of the swing adsorption whereby completion time for regeneration of an adsorber bed can be changed to match completion time for concurrent adsorption by the on-stream bed to be replaced on-stream with the regenerated bed. It is an object of the present invention to provide such control so that the system can be operated under optimal conditions for the normal concentration of adsorbate in the feed gas but the regenerated bed made available for use more quickly than normal to accommodate for a reduced on-stream time resultant from increased adsorbate concentration above the normal level. Compared with convention system design providing operation to accommodate the highest expected adsorbate concentrations, this would permit longer on-stream time for normal operation while permitting reduction in on-stream time to accommodate for higher than normal adsorbate concentrations, or sudden unexpected changes to adsorbate concentration without limitation by the time required to complete regeneration of the replacement bed.